Story highlights

Syrian rebels call them "wizwayzi," humming eyes in the sky they say guide the government warplanes and gunners who bombard their positions.

They're surveillance drones, easily visible from the ground and seen in video shot by rebel fighters. Syrian opposition activists and rebel commanders say both the tempo and accuracy of airstrikes have increased since government forces began using them -- and they point to Iran as the source of the equipment.

"Of course it's Iranian. It doesn't go up that high. We can see it," Muhiyee Deen al-Zein, a Free Syrian Army leader in Homs, told CNN.

"They say, 'The wizwayzi is out.' It's a small drone that films al-Qusayr, and then you think, 'Oh God, help al-Qusayr, it will sleep under rockets.' "

Opposition activists say the number of airstrikes has gone up sharply since the collapse of a cease-fire for the Muslim holiday of Eid al-Adha, growing from about 20 a day before the truce to as many as 60 a day this week.

Wednesday, Russian-built MiG jets pounded targets in the Damascus suburbs, near the port of Latakia and around the cities of Deir Ezzor and Idlib, according to the Local Coordination Committees of Syria, a network of opposition activists.

Syria is Iran's leading ally in the region. The Islamic republic has said it doesn't supply military aid to the government of Syrian President Bashar al-Assad, which is battling a 19-month-old revolt, but Iran hasn't commented on the drone allegations.

Defense analysts say the aircraft displayed by the rebels resemble Iranian drones, but cautioned that they could be training craft or even fakes.

And at the United Nations, British diplomats said they are "deeply concerned by credible information that Iran is providing military support" to al-Assad.

But there's no smoking gun connecting Tehran to the recent airstrikes -- leaving only a distant, suspicious hum in the air over Syria.